visible() on empty <div> is false; prevents valiation of Palette component

Noticed that required validation for Palette was not working; a little debugging shows that the validation disabled because the <select> element is not deepVisible().

The enclosing div.t-palette is not jQuery.is(":visible"); that's because it has no text content (I believe), just a hidden field and nested divs.

The correct solution is that visible() should be based on the CSS "display" property not being "hidden".